1. Field of the Invention
The present invention relates to an automotive in-cabin audio system or, in particular, to an improvement of the audio effect of an automotive in-cabin audio system in which an independent signal is sent to each of a plurality of speakers in the cabin from a multichannel player, that can reproduce the music sources of a multichannel recording system, such as a DVD (digital versatile disk) player.
2. Description of the Related Art
Conventionally, vehicles such as automobiles are equipped with an audio system for enjoying music in the cabin. In early stages of development, a simple two-channel stereophonic system comprising two speakers arranged on the left and right side was the mainstay of the audio system installed in the cabin. The demand for enjoying better music in the cabin, however, has promoted the extension of an in-cabin audio system with an increased number of speakers installed in the cabin.
An in-cabin audio system having six speakers is an example. The six speakers include, for example, a center speaker installed at the front center in the cabin, a left front speaker and a right front speaker installed on the front left side and the front right side in the cabin, a left rear speaker and a right rear speaker installed on the rear left side and the rear right side in the cabin, and a woofer. As an advanced form of this 6-speaker system, a tweeter is added to each of the left and right front speakers. In this audio system, the sound is reproduced from each speaker by correcting the frequency characteristic and processing the reverberation of the sound distributed to the left and right channels. In the process, the sound heard by an occupant in a rear seat, for example, produces an audio effect based mainly on the sound output from the rear speakers.
Recent years, on the other hand, have seen the extension of a multichannel audio system with an increased number of channels for music sources in place of the two-channel music sources such as the audio cassette and CD. A DVD system employing an independent 6-channel or 8-channel digital surround system is an example of the multichannel audio system.
In the DVD digital surround system, the music sources are reproduced from six speakers arranged in the cabin from the disks in which the music sources are recorded for the six speakers by independent multichannel recording systems. The six speakers for the DVD system include a center speaker, a front left speaker, a front right speaker, a rear left speaker, a rear right speaker and a woofer. The sound reproduced from the woofer is a low-pitched sound of not higher than 120 Hz and has a smaller amount of information than the other channels reproducing up to 20 kHz. The six channels in the DVD, therefore, are called 5.1 channels.
If it is assumed that the six speakers described are mounted in an automobile, in the case where an occupant (listener) is seated in the rear right seat, for example, the same sound is reproduced basically from the front right speaker and the rear right speaker of the conventional two-channel stereo system. In the case where the same sound is reproduced from the front right speaker and the rear right speaker, therefore, the sound heard by the listener seated in the rear right seat is governed by the rear right speaker.
In the case where the sound of 5.1 channels described above is reproduced from the six speakers, on the other hand, the sound reproduced from the five speakers other than the woofer can be freely controlled in view of the fact that the sound of the six channels are recorded in the recording media independently. The sound can run around the listener, for example, in the case where the same sound is reproduced from the five speakers each with a predetermined time lag.
In the case where a music source is reproduced in such a manner that the sound runs around the listener as described above, however, the problem is that the sound heard by the listener seated in the rear right seat from the front left speaker far from him is low in pressure with the high-pitched sound attenuated while the sound heard from the rear right speaker is high in pressure with a wide bandwidth with the result that the same sound is heard as a different sound for the listener.
This problem arises from the difference of the sound pressure and the frequency characteristic between the sound reproduced from the speakers far from the listening position and the speakers situated near the listener who listens to the independent sounds of many channels of multichannel sources reproduced from a plurality of speakers. In the case where the sound reproduced from the front speakers and the rear speakers mounted on the vehicle are different, for example, the sound originating from the front speakers and the sound originating from the rear speakers are heard differently by the occupant of the rear seat. In such a case, the sound coming from the front speakers is lower in sound pressure than the sound from the rear speakers and tends to have a deteriorated frequency characteristic, especially in the high frequency range.